1. Field of the Invention
This invention relates to photographic lenses and, more particularly, to zoom lenses suited to video cameras.
2. Description of the Related Art
The video camera employs, as its image pickup element, a CCD, MOS, etc. This type of image pickup element has, however, a high reflection factor of the cover glass in front of it, or the image receiving surface thereof. Therefore, since reflected light from the image pickup element (image plane) goes back into the photographic lens and is reflected again backward from the lens surfaces to be incident on the image pickup element, a series of so-called ghost images are formed on the image pickup element.
When the reflected light from the lens surface is focused at or near the image pickup element, it forms the ghost images of the object being photographed. On the other hand, when such reflected light is focused at an appreciably long distance from the image pickup element, it becomes undesirable light which is called flare.
For example, the reflection, in turn, from the image pickup element surface and the photographic lens surface which, though putting the focus of the reflected light more or less away from the image plane, brings it behind the image plane gives rise to the undesirable light with high possibility when the aperture stop is closed down.
From this point of view, the lens for the video camera is required not only to be well corrected for aberrations, but also to be free from the undesirable light. As the minimization of the size of zoom lens advances, the difficulty of lens design for both good correction of spherical aberration, coma and distortion and sufficient reduction of the undesirable light increases. This leads to a lowering of the contrast of the image.
Meanwhile, zoom lenses suited to video cameras are known in U.S. Pat. Nos. 4,618,219, 4,621,905, 4,525,036, 4,659,187, 4,653,874, and 4,518,228. In addition, there are Japanese Laid-Open Patent Application Nos., Sho 59-222807, 60-260912 and 61-20291 U.S. patent application Ser. No. 190,472 filed May 5, 1988 now U.S. Pat. No. 4,812,024.